Implement guardian unique perks and fix EXECUTIONER

CRITICAL FIXES:
- Add EXECUTIONER to SPECIAL_EFFECTS constant (was used but not defined)
- Implement all 10 guardian unique perks when pacts are signed:
  - Floor 10: Fire spells cast 10% faster
  - Floor 20: Water spells have 10% lifesteal
  - Floor 30: Air spells have 15% crit chance
  - Floor 40: Earth spells +25% damage to guardians
  - Floor 50: Light spells +20% damage
  - Floor 60: Dark spells have 20% lifesteal
  - Floor 70: Life spells heal 30% of damage
  - Floor 80: Death spells execute below 20% HP
  - Floor 90: Void spells ignore 30% resistance
  - Floor 100: All spells +50% damage and cast 25% faster

- Add getGuardianPerks() function to compute active perks from signed pacts
- Apply guardian perks in combat calculations (damage, lifesteal, crit, execute)
- Import getGuardianPerks in store.ts

🤖 Generated with [Claude Code](https://claude.ai/code)

src/lib/game/upgrade-effects.ts

@@ -80,6 +80,7 @@ export const SPECIAL_EFFECTS = {
   BERSERKER: 'berserker',                // +50% damage when below 50% mana
   COMBO_MASTER: 'comboMaster',           // Every 5th attack deals 3x damage
   ADRENALINE_RUSH: 'adrenalineRush',     // Defeating enemy restores 5% mana
+  EXECUTIONER: 'executioner',            // Instant kill enemies below 25% HP
   
   // Study special effects
   QUICK_GRASP: 'quickGrasp',             // 5% chance double study progress per hour